An included angle is the angle between two sides of a triangle.

Step 1: draw the isosceles triangle △RST

**Step 2: find the measure of ∠S

Since $\overline{\rm RS}=\overline{\rm RT}$, it follows that ∠S = ∠T.

Let ∠S = ∠T = x°:

$$ 50°+x+x=180° $$

$$ \therefore x=\frac{180°-50°}{2}=65° $$